1. Field of the Invention
This invention relates generally to oil well drilling systems and more particularly to a drilling fluid flow monitoring and control system for use during drilling of wellbores.
2. Description of the Related Art
Oilfield wellbores are formed by rotating a drill bit carried at an end of a drill string. The drill string is comprised of a tubing which may be drill pipe made of jointed sections or a continuous coiled tubing and a drilling assembly that has a drill bit at its bottom end. The drilling assembly is attached to the bottom end of the tubing. To drill a wellbore, the drill bit is rotated by a mud motor carried by the drilling assembly and/or by rotating the drill pipe. A drilling fluid, also referred to as the "mud," is pumped under pressure from a source at the surface (mud pit) through the tubing. The mud serves a variety of purposes. It is designed to provide the hydrostatic pressure that is greater than the formation pressure to avoid blow outs. The mud drives the drilling motor (when used) and it also provides lubrication to various elements of the drill string. Commonly used drilling fluids are either waterbased or oil-based fluids. They also contain a variety of additives which provide desired viscosity, lubricating characteristics, heat, anti-corrosion and other performance characteristics.
The mud discharges at the bottom of the drill bit and returns to the surface via the annular space between the tubing and the wellbore inside (also referred to herein as the "annulus"). One of the primary functions of the drilling fluid is the control of the formation pressure. The hydrostatic pressure exerted by the mud column is the primary method of controlling the pressure of the formation. Whenever the formation pressure exceeds the pressure exerted by the fluid column, formation fluid enters the wellbore, causing a "kick." For the purpose of this invention, a kick is defined as any unscheduled entry of formation fluid into the wellbore. Early detection of kicks and prompt initiation of control procedures are keys to successful well control. If a kick is not detected early enough or controlled properly when detected, a blowout can occur. One method to determine the presence of a kick is to accurately determine the amount of the drilling fluid being supplied to the drill string and the amount of the fluid returning to the surface. Gain in the fluid quantity is an indicia of a kick. During the drilling of a wellbore, a certain amount of the drilling fluid is lost because it penetrates into the formation. However, excessive loss of fluid can be an indicia of wash outs.
Fluid flow measurements are made at the surface for determining the amount of the drilling fluid entering the drill string and the drilling fluid returning to the surface. Such measurements are then used to determine certain operating parameters, such as kick, fluid loss etc. There are two modes relating to the pumping of the drilling fluid into the well: (1) pumping fluid by main mud pumps through the drill string during the drilling operation, and (2) pumping fluid by a secondary into the annulus during tripping operation when the main mud pump is inactive. Fluid is pumped in the latter case to compensate for the reduction in the volume due to the removal of the drill pipe and to ensure the maintenance of the required hydrostatic pressure. A common method to determine out flow (flow rate of the returning fluid) is to install a flow sensor in a bypass line made from the main return line. Solids from the returning fluid tend to settle in the bypass line, which frequently inhibits the measurements. Electromagnetic sensors are commonly used to determine the flow of the fluids. Such sensors provide relatively accurate measurements for water-based drilling fluids. For oil-based muds, especially muds containing a relatively high percentage of oil, such sensors do not provide accurate measurements. Other commercially available sensors have not gained much use in oilfield applications because they are either prohibitively expensive or do not provide accurate measurements. The present invention addresses some of the problems with the flow measurement apparatus and methods and provides a prior art flow measurement system that provides relatively accurate measurements for any type of drilling fluid, including oil-based and water-based muds. The present invention also provides novel sensor arrangements for determining fluid flow rates.